1. Field of the Invention
This invention concerns production of a thermostable crosslinked PVC copolymer. The copolymer comprises a comonomer with a hydrolyzable substituent. This substituent is a member of the group of hydrolyzable alkoxysilanes, and will from now on be called the silane group.
2. Description of the Related Art
In literature there is described several methods for production of crosslinked PVC, but only few of these are in use in an industrial scale today. Radiation is one of the techniques most used for crosslinking of PVC. Other methods are based on for example multifunctional plasticizers which are activated by radical initiators. Further, blocked polyisocyanates can be used as crosslinking agents and different polyfunctional thiols would also give crosslinked PVC.
Some of the disadvantages with these systems are that often very poor thermostability is obtained for the products, discolouring as a result of degradation, increased brittleness and too quick crosslinking which also have influence on the ability to form the products and their appearance.
Lately the patent literature has described processes where bifunctional silanes are used as crosslinking agents (DE 3 719 151, JP 55 151 049, NO 166 189). These processes are different from the above described in that processing and crosslinking is separated. Bifunctional silane crosslinking agent is added to the compound. During processing and at high temperature (140-190.degree. C.) the crosslinking agent reacts with the polymer while the silane-groups remain intact. After shaping of the product the water/steam treatment will result in that the silane groups are hydrolyzed and crosslinked. This technique has clear advantages in preference to those where crosslinking is taking place during processing as the processes can be better controlled and the time for crosslinking can be determined.
These methods however also have their limitations. When using .gamma.-mercaptopropylsilane as crosslinking agent as described in DE 3719151 a lead stabilizer is needed to obtain crosslinking. This a clear disadvantage for the environment. In this patent there is used time periods of 6 hours to obtain sufficient crosslinking. This is an unrealistic long time and will be commercially unfavourable. Further it is a fact that yellow complexes are formed which characterize the products. This own colour of the products make it difficult to make manufactured products with pigmentation as wanted.
In Norwegian patent application No. 912341 however there is described how to prevent yellow colour by addition of low molecular epoxy resin. It is also possible to use aminosilanes instead of mercaptosilane to avoid the problems with colouring of the plastic. Further, by use of aminosilanes as a crosslinking agent one proceeds with separation of processing and crosslinking. A weak point however is that amines generally have negative influence to the heat stability of PVC.